This invention relates generally to the field of thin film processing through the use of a direct current (DC) plasma. Specifically the invention relates to applications of such processing for which very accurate control of the processing plasma is required. It also discloses a new switch-mode power supply design as is often used in such applications.
The field of DC plasma processing for thin film applications is one which is well known. In these processes, a DC power supply creates an electric potential which causes the existence of a plasma. The plasma then acts upon some material so as to cause processing of an item or substrate. This processing may be the deposition of some substance on the substrate. It may also be some type of etching or conditioning of the surface of the substrate. Naturally, both the materials, substrates, and the nature of the processing can vary greatly. Such applications may range from the creation of microchips to the aesthetic coating of items.
One of the unique challenges in this field is that it can require an unusually precise control of the electrical power applied to the plasma. Unlike other fields where loose control or even the mere existence of the plasma is sufficient to achieve the desired goal, the fact that thin film processing can be as subtle as the creation of a single atomic layer has resulted in unusually strict power control requirements. For this reason, the techniques and systems utilized in other applications (such as those involved in X-Ray generation, evaporative processing, and the like) have not been considered pertinent to this unique area.
One of the ways in which those skilled in the field of DC plasma thin film processing have achieved the degree of power control required has been through the use of a specific type of power supply known as a switch mode power supply. In such a device, the DC power source is seemingly redundantly converted to an AC signal and then reconverted back to a DC signal. By making the intervening AC signal have a higher frequency, relatively quick and accurate control can exist. In addition such switch mode power supplies are often used in this unique field because of the low stored energy necessary to smooth the ripple in the final waveform compared to energy stored in conventional power supplies.
Low stored energy has become an important facet of the DC plasma processing field because the plasma often has arcs occur within it. As such plasma processing applications have become more demanding, the existence of and impacts of arcing within the plasma have been magnified. When arcs and other lower impedance conditions occur in the processing plasma, application of all the energy stored in the output circuitry of the power supply can be discharged into the arc. This can not only cause damage to the system, but it can create defects in the eventual product. Further, these arcs may require the immediate shut off and slightly delayed reinstitution of the application of the power. The amount of time necessary to discharge and then re-charge traditional filter components before reapplying the power is often unacceptable. Thus in general, by lowering the amount of stored energy necessary to smooth ripple, both the system and the end product can be improved.
As mentioned, switch mode power supplies are often used in such applications because they not only offer rapid control, but they can also store lower amounts of energy. The processing demands have evolved, however, to a point at which even the enhanced performance of traditional switch mode power supplies is not enough. The amount of time it takes to stop and restart the application of power, the amount of ripple existing, and even the reduced amount of energy stored in such power supplies has become unacceptable for many applications. As a result those skilled in the art of thin film processing through the use of a DC plasma have sought to improve the switch mode power supplies used. Generally this has been directed toward increasing the frequency at which the intermediate alternating signal is generated. Unfortunately, this direction of pursuit has not always met the needs of the users for a number of reasons.
First, increasing the frequency of the intermediate alternating signal is practically limited by the speed at which the switches which generate the alternating signal can operate. While faster switches are continually developed, they can be overly expensive or can have limits to the amount of power they can transmit. Perhaps even more fundamentally limiting however, is the fact that the utilization of a filter to reduce ripple is counterproductive since such filtering is accomplished through a storage of energy. This energy still reaches the plasma after the system has been shut down. While those skilled in the art have known this aspect, it has previously been viewed as a necessary evil to creating a low ripple output.
Both the amount of energy stored in such filter components and the amount of time needed to charge or discharge these components have generally been viewed as virtually a physical limitation which was immutable. Thus, those skilled in the art had most often approached the need for more accurate energy control by increasing the frequency of the switching circuits in the switch mode power supply. Since this type of approach does have the incidental effect of reducing the amount of energy needed to be stored in the filter elements of the output circuits, this type of solution was generally preferred. As this frequency has increased, however, the cost of the power supply has increased. Further, parasitic elements within the supply have become increasingly important and have further added to the cost of the construction of the supply. There are also limitations on how high the frequency can be extended due to the lack of availability of components capable of high power operations. In addition, power supply losses have also increased due to intrinsic limitations of practically available components. For these reasons the mere increase in the frequency of the switch mode power supply has not been adequate to meet the more demanding needs.
In sharp contrast, the present invention takes an entirely different approach and in fact shows that the amount of energy stored in an output stage filter is not an immutable given but can be extremely reduced or even eliminated through a different approach to the problem. Filtering need not be viewed as a necessary evil but rather it is a component which can even be totally eliminated from some circuit designs. The approach taken by the present invention is surprising not only from the perspective of the preconceptions that those in the field had, but also as a result of the fact that it utilizes technology and elements which had been long been available to those skilled in the art. While those skilled in the art had long felt the need to overcome these problems, they did not realize the possibility existed to use technology developed for another purpose (power transmission over long distances) which could be applied to this problem. This could be because the preconceptions and approaches typically used by those skilled in this art may have actually acted to teach away from the direction of the present invention.